Varying supply-demands for distillate fuels and light olefins like propylene and ethylene, are affecting the oil refiners worldwide. Several drivers, like increasing gap between demand and supply for propylene are affecting the growing need for the production of the same by Fluid Catalytic Cracking. The need for propylene is growing faster than that of ethylene, while on the other hand the co-production of propylene from steam crackers (˜70% of supply) is expected to decline as plants are optimized to produce higher-value ethylene. The bulk of the additional propylene will need to be produced from changing the ratios of FCC product streams. Shifts in transportation fuels and changes in fuel's specifications are having a great impact on the refinery. With the increasing growth rates being observed in Asian countries like India and China, demand for middle distillates, which being the major mass transportation fuel is increasing at a higher rate than that for gasoline. Also, in Europe for instance due to the growth of diesel consumption there are several refineries, which are attempting to reduce their gasoline yield because of imbalance over supply. Addition of new technologies in FCC will be needed to further increase the propylene production without compromising the yields of middle distillates.
FCC process involves contacting and cracking a heavier hydrocarbon feed like vacuum gasoil, atmospheric tower bottom, vacuum residue etc. in a reaction chamber with a hot regenerated catalyst in a fluidized condition and removing the products from the deactivated catalyst to yield desired products like LPG, gasoline and middle distillates etc. Catalyst is deactivated due to coke deposition which can be regenerated by burning with air or any oxygen containing gases in the regenerator. With the varying market demands and scarcity of light crudes put immense pressure on refiners to increase the flexibility of the fluid catalytic cracking process to be able to maximize the yield of the desired products. Those who are skilled in the art of FCC can easily understand the design and operational limitations of single stage FCC process. The two-stage processing of hydrocarbon feeds in FCC is used with various objectives, like processing of heavy feeds, maximization of desired products, increasing the quality of the products and scores over the single stage process in every aspect.
U.S. Pat. No. 3,803,024 describes a two stage catalytic cracking configuration, with a common fractionator to increase the product yields. Fresh feed being introduced into a first catalytic cracking zone, employing an amorphous Silica Alumina catalyst and the partially converted material being separated using a fractionator and reintroduced into a second catalytic cracking zone employing a zeolite catalyst to get the desired conversion. Unconverted material from the common fractionator is recycled to any of the two reactors. The recycle of heavier bottom fractions from the common fractionator results in the buildup of refractory material in the system.
U.S. Pat. No. 5,009,769 describes a parallel two riser system with single reactor stripper with two stage regeneration for converting different types of hydrocarbon feedstocks to light olefins such as propylene. Fractionation of the reactor effluent is carried out in single separation column and naphtha & light cycle oil range hydrocarbons are further cracked in one of the risers. Regenerated catalyst is fed to both risers independently. Here the unit can be tuned to treat a variety of feed qualities.
U.S. Pat. No. 6,287,522B1 describes a process for the dual riser contacting of a primary feed and a secondary recycle feed fraction with independent recovery of the separate streams from the riser cracking zone to improve the product yields and properties. In one of the embodiments, spent catalyst is recycled to one of the risers, to crack fresh feed. The main disadvantage of this process is that the catalyst activity reduces considerably, after passing through one riser and the same catalyst may not be effective in cracking reactions taking place in the second riser.
U.S. Pat. No. 7,491,315B2 describes a dual riser FCC reactor process with light and mixed light/heavy feeds to increase the yield of light olefins. Same catalyst is being circulated in both the riser reactors. The two reactors can be operated under different operating conditions. Coke precursors, which may be a heavy feed, are to be added to the lighter feed to increase the coke make for the proper heat balance of the unit. In all the above mentioned two stage systems employing dual riser reactors, problems like back mixing and higher coke yield persist.
In order to avoid back mixing in the prior art riser, a down flow FCC reactor (hereinafter downer) had been proposed in U.S. Pat. No. 4,385,985. Applications of downer reactor and its applications are given in U.S. Pat. No. 7,153,478 B2. Pilot plant studies conducted by M. A Abul Hamayel (Petroleum Science & Technology, 2004, Vol. 22, No. 5&6, 435-490) show that higher yield of propylene and gasoline obtained from a downer, compared to a riser. Overall coke yield (wt % fresh feed) was also found to be lesser in case of a downer. However, for making use of the advantages of the downer reactor, proper initial contact of the catalyst and feed is very important.
A riser downer coupling reactor has been proposed recently by Fei Liu et al (Ind. Eng. Chem. Res, 2008, Vol. 47, 8582-8587) where, the regenerated catalyst enters at the bottom of the riser reactor and mixes with a fresh hydrocarbon feed and flows upwards and the flow is diverted at the riser top, into a downer reactor to complete the reaction. Changning et al (Chem. Eng. Technol. 2009, Vol. 32, No. 3, 482-491) suggests a downer to riser coupling reactor, where the fresh feed and regenerated catalyst is mixed in the inlet of the downer reactor and flows downward. The downer reactor is connected with a larger diameter riser reactor with a U tube bend, where steam is injected to assist the upflow of the catalyst in the riser reactor.
Chinese Patent No. CN101210191A proposes a similar configuration where the downer and riser reactors are connected in series wherein the hydrocarbon feed is introduced into the inlet of the downer reactor for catalytic cracking at a Catalyst/Oil ratio of 5-40 and operating temperature of 480-660° C., the entire reactor effluent is further contacted in a riser with the spent catalyst from the downer at a Catalyst/Oil ratio of 10-35 and operating temperature of 450-650° C. The disadvantages of such systems are (i) significant reduction in conversion in the second reactor due to use of partially deactivated catalyst from the first reactor; (ii) cracking of the desired product fractions formed in the first reactor. Furthermore, simultaneous maximization of middle distillates and light olefins is not possible using such configuration.
U.S. Pat. Nos. 6,641,715 and 7,220,351B1 describes method and device for catalytic cracking comprising reactors with descending and ascending flows. In U.S. Pat. No. 6,641,715, either recycle or a mixture of fresh feed and recycle feed and regenerated catalyst enters the downer reactor, the cracked gases are separated from the coked catalyst in a first separation zone and the coked catalyst is reintroduced into the lower portion of the riser reactor. The said catalyst and the fresh feed are circulated, the used catalyst is separated from the riser effluent stream, in a second separation zone and it is recycled into regeneration zone consisting of one or two regenerators. A non negligible amount of catalyst will be partially deactivated during the passage through the downer reactor, which reduces the extent of cracking in the riser reactor.
U.S. Pat. No. 7,220,351B1 also describes a similar method, except the use of regenerated catalyst in both reactors. Here the riser is a conventional riser, operating at conventional cracking conditions. The production of olefins and in particular propylene by recycling the gasoline or only a fraction of gasoline produced in the riser to downer.
US Application 2008/0011644A1 describes an ancillary cracking of heavy oils in conjunction with conventional riser FCC unit, using a downer reactor. Here, the production of light hydrocarbons consisting of ethylene, propylene, and butylenes and gasoline is enhanced by introducing heavy oil feed stream derived from an external source into an ancillary down flow reactor that utilizes the same catalyst composition as the FCC unit nearby.
In the above mentioned process scheme, same catalyst is being used in the two reaction zones, namely downer and riser; this makes it less flexible for the processing of feed stocks of widely varying quality.
From the prior art, it can be observed that, researchers have suggested process schemes to maximize the yields and selectivity of desired products like middle distillates, gasoline, propylene etc. Market demand for propylene as well as middle distillates, are increasing worldwide. Therefore, there is a requirement for a process which can simultaneously maximize middle distillates, propylene and ethylene, in order to cater to the increase in demand of both the products.
It is desirable to have an improved FCC process and apparatus for maximization of light olefins including ethylene and propylene and middle distillates, with flexibility of alternate mode of operation for maximization of gasoline.
The present invention relates to a novel process and apparatus of FCC which provides for maximization of light olefins including ethylene and propylene and middle distillates yield, with flexibility of alternate mode of operation for maximization of gasoline.
The invention is aimed at meeting the changed needs of the present demand trend. The invention also discloses suitable apparatus required for the invented process. Refineries must augment their production to be able to be in step with the existing demand of the products. As the present trend shows increasing use of light olefins, their production must be increased economically. The invention additionally offers maximization of gasoline yield. The invention discloses a two stage fluid catalytic cracking process and an apparatus for the same.